Indian Space Research Organisation (ISRO) is developing a semi-cryogenic engine that will use eco-friendly kerosene as propellant. The advantage of using this semi-cryogenic engine in the launch vehicle is that it uses refined kerosene which is lighter than liquid fuel and can be stored at a normal temperature. The current fuel — a combination of liquid hydrogen and liquid oxygen—is heavier than kerosene and has to be stored at freezing temperatures of (-)253 degree Celsius.

According to some agency reports, if things go as planned, Isro will flight-test the semi-cryogenic engine by 2021.

Speaking to TOI, Dr K Sivan, director of Thiruvananthapuram-based Vikram Sarabhai Space Centre, said, "Kerosene is a fuel lighter than the conventional combination of liquid hydrogen and liquid oxygen but it gives higher thrust. Therefore kerosene occupies less space and more propellant can be packed in the semi-cryogenic engine's fuel compartment. However, liquid oxygen will be retained as oxidiser. The advantage of using this semi-cryogenic engine is the payload capacity of the launch vehicle will increase from four tonnes to six tonnes. This kind of rocket with the semi-cryogenic engine can therefore take heavier satellites into space and can also be used for interplanetary missions and deep space missions."

"We will only replace the second stage of the launch vehicle like GSLV Mk-III, which now uses liquid fuel, with the semi-cryogenic engine. The rocket will retain the cryogenic upper third stage," he said.

"Space agencies of many countries like the US and Russia have been using the semi-cryogenic engine as it gives high thrust. In fact, Falcon-9 of SpaceX (the first US commercial company whose rocket has made multiple flights to the International Space Centre) has also been using the semi-cryogenic engine technology," Dr Sivan said, adding "various tests on the engine are in progress".

The Union Cabinet had cleared the semi-cryogenic engine project in 2008 at an estimated cost of Rs 1,798 crore. Although the idea was to develop the technology in 2014, the project got delayed.